We continuously experiment with reminders to find the best time to send them, but currently, they are sent out after four days to all recipients who have not listened to our downloaded at least one track on the campaign. We want to eventually allow users to adjust this setting themselves in the future and to automatically throttle how often a recipient receives emails (while prioritising new promo sends). It is not available at this time.
To turn reminders on we recommend you do this during the campaign creation. Please find the option to enable them at the bottom of the campaign editor.